IT is a curious fact that there appears to be a certain reluctance in this country to use the members of the Mollusca as tood. Oysters, it is true, are not disdained, but whelks and mussels are by no means fashionable dishes, while snails are regarded as quite outside the pale of human consideration, although they are eaten with relish in other countries. Yet although no one would think of scorning crab, lobster, or prawn, these in fact rank very little higher in the animal scale than the Mollusca.
There can be little doubt that the Romans were particularly partial to snails—not to the common or garden snail, but to a larger variety, known as the Roman snail or apple-snail, found in abundance in certain parts of England. This has led to the belief that these large snails were introduced into the country by the early invaders. As is usually the case where information is uncertain, legend steps in with an enthusiastic if misguided attempt to fill in the gaps.
The origin of the belief appears to be two-fold. In the
first place, in excavations of the sites of Roman camps and dwellings, the empty shells of this particular snail are often found associated in quantities with the remains of Romano-British pottery. Secondly, the snails are usually found to-day within a fairly short radius of a Roman site, implying, it may be supposed, that the descendants of those snails that survived the gustatory onslaughts of the early invaders still cling to the ancestral haunts.
In this country, the Roman snail is confined to the southeast corner of England, and even here is local in its distribution. Usually it is found only on chalky soil, though sometimes thriving on sand, and when it is remembered how thickly the chalk areas of the south-eastern counties are studded with Roman remains, it is easy to see that no local patch of snails could be far from one of them. To associate them with the Roman occupation, then, was natural and the idea once having been started, an observer would be influenced to search more closely for them in such areas, overlooking them, as a consequence, in those areas where no Roman relics had been found.
Fortunately, the geologist is at hand to give us enlightenment in the matter. The Romans may have introduced some snails to augment the numbers already here, or even to replenish depleted stocks, but Helix pomatia, the Roman snail, was here before Cajsar cast covetous eyes on the cliffs of Britain. Their fossil remains are associated not only with eoliths, that is with the very earliest flint tools fashioned by man, but also with deposits of earth that were probably laid down before man had even begun to throw off his brutish mantle. If any further proof that the Romans were not responsible for their introduction were needed, it is that the snails are not found around the sites of Roman camps elsewhere in this country than in the south-eastern counties.
The Roman snail is sometimes called the apple-snail because it is wrongly assumed that pomatia in its Latin name Helix pomatia means apple. Actually, however, it is derived from a Greek word, poma, meaning a pot-lid. In winter, or in very dry weather, all snails tend to bury themselves under dead leaves or decaying vegetation, or to creep into a cleft in a rock, under stones or, indeed, under or into anything that will give them shelter from the desiccating powers of the sun’s rays. In winter, or in prolonged drought, the garden snail not only buries itself but, to preserve what little moisture
it can and to prevent undue evaporation from its own body, closes the opening of its shell with a film of mucus, which hardens on contact with the air. Snails may often be turned out in the garden with the shell sealed up in this manner. A small aperture is left in the film to admit air, but otherwise the snail is well-protected both from the elements and from its smaller enemies. The Roman snail, under similar circumstances, also withdraws into its shell and seals the opening, but this it does with a thick, chalky plate, the pot-lid—not perforated to admit the air but porous like plaster of Paris, which it closely resembles.
Just as in the higher vertebrates, including man himself, provision is made for protecting the more vulnerable organs, such as heart and lungs, with a strong skeleton, in their case the ribs, so in the Mollusca these organs are always found under the shell. For example, in the snails the lungs and other organs are permanently lodged within the shell, so that not only can the body be withdrawn into the shell at the alarm, but the vital organs are always protected by it. The carnivorous slug, on the other hand, has so small a shell, which it carries at the hinder end of its body, that it could not withdraw the whole of the body into it. But we find that the pulmonary chamber, heart, and other organs are immediately underneath it. Further, the vegetarian slugs that harass our gardens, and which appear to be devoid of a shell, have a shell of sorts hidden underneath the warty skin situated near the front end of the body and under this, too, the vital organs are lodged. In the last-named, owing to the lack of an external shell, we may see the opening to the lung, or pulmonary chamber, on this same warty patch of skin, varying in position according to the species examined.
All marine molluscs breathe by gills, but all land molluscs, and practically all of those living in freshwater, have a pulmonary chamber; this is not a lung in the sense of a spongy mass of tubes such as is found in the higher animals but merely a cavity with walls richly supplied with blood-vessels and opening to the exterior by a single aperture.